Phosgene is used in industrial scale as an important starting compound for the production of diisocyanates and polycarbonates among others. A need for phosgene substitutes has arisen as a result of its high toxicity on the one hand and the highly restrictive legislative safety regulations brought about by this with regard to transport, storage and use on the other hand. This need is covered by diphosgene (trichloromethyl chloroformate) that is liquid at standard conditions and crystalline triphosgene (bis(trichloromethyl)carbonate) [H. Eckert, B. Forster, Angew. Chem., 99 (1987) 922-23; Angew. Chem. Int. Ed. Engl., 26 (1987) 894-95; F. Bracher, T. Litz, J. Prakt. Chem., 337 (1995) 516-18; L. Cotarca, P. Delogu, A. Nardelli, V. Sunjic, Synthesis, (1996) 553-76].
In practice, it has been determined that, as was previously the case, it is advantageous to use gaseous phosgene in chemical production processes. Reasons for this are, for one, that known methods can be run with existing plants and, for another, the fact that work is frequently done with an excess of phosgene that has to be removed after the reaction. However, the separation of excess phosgene mentioned in the latter case often turns out to be difficult with less volatile phosgene substitutes, whereas gaseous phosgene can be easily removed [J. S. Nowick et al., J. Org. Chem., 61 (1996) 3929]. However, as a result of the above mentioned legislative safety regulations, phosgene itself is no longer commercially available. Hence, a need exists for a harmless method of production of pure phosgene,
immediately before its use in the reaction, from stable precursors such as the substitutes diphosgene and
especially triphosgene via their regulated and controllable reaction to phosgene.
Such a reaction of diphosgene and triphosgene on reaction catalysts is already known, but serious disadvantages exist with the known reaction catalysts: thus, triphosgene is stochiometrically reacted on metal salts with strong Lewis acid characteristics, such as aluminum chloride or iron chloride, to phosgene, carbon dioxide and carbon tetrachloride according to the following equation [L. Cotarca, Synthesis, (1996) 556]:
Cl3Cxe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94CCl3xe2x86x92COCl2+CO2+CCl4.
In this case, the yield of phosgene is only a third of the theoretically possible value therewith. Moreover, the resulting side-products can be disturbing in the subsequent reactions of phosgene and the conversion reaction runs uncontrollably to a great extent. On the other hand, triphosgene is completely stable against weaker Lewis acids such as titanocene dichloride and zirconocene dichloride.
Triphosgene can also be reacted to phosgene on activated charcoal. Although the reaction here is nearly quantitative, the reaction is uncontrollable and can even take on an explosive-like character.
Diphosgene and triphosgene can also be reacted to phosgene on Lewis bases such as pyridine, but in this case, the extremely fast conversion reaction is also not controllable.
In light of this background with the above mentioned disadvantages of the known methods for reacting diphosgene and triphosgene to phosgene, the problem of the present invention is to provide a method for the controllable and substantially quantitative production of phosgene from diphosgene and/or triphosgene.
It was surprisingly found that this problem can be solved by reacting diphosgene and/or triphosgene to phosgene on a catalyst comprising one or more compounds with one or more nitrogen atoms with deactivated free electron pair. Triphosgene can be used in the form mentioned above (bis(trichloromethyl)carbonate) as well as in the cyclic form given in the following formula: 
In a preferred embodiment, the deactivation of the free electron pair of the nitrogen atom occurs by mesomerism and/or one or more electron-attracting and/or space-filling groups in the vicinity to the nitrogen atom. The term xe2x80x9cin the vicinityxe2x80x9d means particularly xe2x80x9cin the xcex1-, xcex2-, or xcex3-positionxe2x80x9d to the nitrogen atom with deactivated free electron pair, particularly preferred is xe2x80x9cin the xcex1-positionxe2x80x9d.
Preferred examples of compounds with nitrogen atom with deactivated free electron pair are compounds with deactivated imine and/or deactivated amine function.
In a preferred embodiment, they are immobilized by binding to polymers such as polyacrylic acid or polystyrene. The immobilized compounds with deactivated imine and/or amine function are optionally bound to the polymer chain over spacer molecules (so-called xe2x80x9cspacerxe2x80x9d. Examples for such spacers are alkoxy groups such as triethylene glycol, tetraethylene glycol and polyethylene glycol groups.
Compounds with deactivated imine function are, for example, higher aromatic or heteroaromatic systems as well as compounds with alkyl groups in the vicinity to the nitrogen atom. Preferred compounds with deactivated imine function that can be used in the method according to the invention are poly-(2-vinylpyridine), phenanthridine as well as phthalocyanine (H2Pc) and metal phthalocyanines (MePc) whose skeletal structure is depicted below: 
The auxiliary group metals of the 4th to 6th period as well as the metals of the 3rd to 6th period of the main groups 2 to 5 are preferred as metal atoms of the metal phthalocyanine, and particularly, the auxiliary group metals of the 4th period (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn).
The metal atoms of the metal phthalocyanine, especially in the case of the auxiliary group metals, can be complexed with one or more additional ligands such as chloro or oxo. The phthalocyanine and/or the metal phthalocyanines can be used in any crystal modifications. Examples for such crystal modifications of metal phthalocyanines are xcex1-CuPc and xcex2-CuPc.
The above-mentioned preferred compounds with deactivated imine function can optionally be substituted on the carbon skeleton. The substituents include alkyl, cycloalkyl, aryl, halogen, nitro, amino, cyano, carboxy, carbalkoxy, carboxamido as well as heterocyclic groups.
In a preferred embodiment, the phthalocyanine or the metal phthalocyanines can be substituted once or several-fold independently of each other on the benzo groups, wherein the substituents are preferably selected from the above mentioned substituents as well as further phthalocyanines and condensed cyclic or heterocyclic compounds that are themselves optionally substituted.
The compounds with deactivated amine function are preferably selected from deactivated tertiary amine compounds. The deactivation occurs, in a preferred manner, by immobilization by binding the amine compound to polymers, wherein tertiary alkylamines are particularly preferred, and the alkyl groups are the same or different and are selected from methyl, ethyl, propyl and higher linear or branched alkyl groups. An example for a catalyst with deactivated tertiary amine function according to the invention is N,N-dimethylaminomethyl polystyrene.
The catalyst for reacting diphosgene and/or triphosgene is preferably used at a concentration from 0.01 to 10 mol %, particularly preferred is from 0.1 to 2 mol %, with respect to the amount of diphosgene and/or triphosgene. If the catalyst is a catalyst immobilized to a polymer by binding of a compound with nitrogen atom with deactivated free electron pair, then the concentration is calculated based on the amount of substance (in mol) of the compounds with deactivated free electron pair bound to the polymer chain.
In a preferred embodiment of the method, this is carried out with diphosgene and/or triphosgene in the liquid state. The reaction temperature is preferably 80 to 150xc2x0 C., more preferably 90 to 130xc2x0 C. and most preferably 100 to 125xc2x0 C.
Although the method according to the invention can be carried out without solvent, it is also possible to use an inert solvent in the reaction of diphosgene and/or triphosgene.
The present invention also provides a device for the production of phosgene from diphosgene and/or triphosgene as reaction material that comprises a storage vessel for diphosgene and/or triphosgene and a reaction chamber, with phosgene outlet, connected to the storage vessel and containing the catalyst.
In a preferred embodiment, the reaction chamber is equipped with heating unit that is optionally electronically controllable. This heating unit can be used in order to optionally melt reaction material transported from the storage vessel to the reaction chamber and/or to accelerate the conversion reaction by an increased temperature.
It is preferred to provide the reaction chamber with a funnel-shaped return flow device discharging into the reaction chamber. In this manner, evaporated or transported diphosgene and/or triphosgene can be condensed in the reaction chamber and led back into the reaction system. In a preferred embodiment, the return flow device consists of a reflux condenser with heat exchanger mounted on the outside of the device or of pitched baffle plates. The baffle plates preferably consist of coated metal plates. The coating can consist of a suitable inert plastic such as polytetrafluoroethylene or perfluoroalkoxy polymer or glass for example. The baffle plates of metal possess a high thermal conductivity such that a thermal equilibrium can be easily set in the reaction chamber.
For reasons of simplicity for delivering the triphosgene into the reaction chamber, the storage vessel is arranged in a preferred embodiment for the up-take of triphosgene in tablet form. In this connection, the loading of the storage vessel can optionally be carried out by means of a tablet magazine, for example, in the form of a tubule of glass or plastic such as PTFE or PFA. In this manner, a simpler loading can be performed without the danger of contact with triphosgene.
FIGS. 1 and 2 show preferred embodiments of the device according to the invention. The device consists of an encased housing (17) of suitable, inert plastic (for example, polytetrafluoroethylene (PTFE) or perfluoroalkoxy polymers (PFA)) or metal with plastic (PTFE, PFA) or glass insert or coating (19). The storage vessel (1) for diphosgene and/or triphosgene (4) comprises a closing cover capable of being fastened (21) or a locking screw (23) on the upper side and a dosage device with closable passage opening (25) to the reaction chamber (5) for the reaction material. The closure (27) of the passage opening (25) can be a flap, cover or a sliding shutter. As a result of the gas development during the reaction, a pressure lock can also be used as a passage opening that permits a higher pressure in the reaction chamber with respect to the storage vessel without phosgene entering the storage vessel with open passage opening, for example, in the transport of the reaction material from the storage vessel into the reaction chamber.
The dosage device can be a simple opening (29) (especially in the case of the use of triphosgene in tablet form (2)) or a motor-driven drive (31) such as a screw drive for example.
In order to avoid moisture in the storage vessel, a dehydration unit (33) with suitable drying substance (for example, silica gel) is mounted in the device in such a manner that it is in equilibrium with the gas volume found over the reaction material in the storage vessel (1).
The catalyst (3) is found in amorphous or crystalline form in the reaction chamber (5), is applied to the walls of the reaction chamber or is immobilized by a carrier.
The reaction chamber is equipped with a heating unit (9) for heating the reaction mixture. A safety valve (35), which is connected over a gas line with an exhaust, is mounted in the reaction chamber directly above the reaction mixture. The return flow device is configured in the form of baffle plates (15) or as a reflux condenser for the phosgene produced (11) with heat exchanger (13). A gas exhaust valve (7) is found at the top end of the reaction chamber (5). The reaction chamber can be cleaned after completion of the reaction with gas, for example inert gas or dry air, wherein the gas exchange can occur over the safety valve (35) and the gas exhaust valve (7).
FIGS. 3 and 4 schematically show special embodiments of the dosage device with which the reaction material is transported into the reaction chamber. In FIG. 3, the delivery of the tablet-formed reaction material (2) from the storage vessel (1) into the reaction chamber (4) occurs according to the xe2x80x9crevolver principlexe2x80x9d with a rotatable disk (37) with at least one passage opening (25), preferably two or more openings, for the up-take of tablets. By turning of the disk (37), the tablets are transported into the reaction room (5) over the heating unit (9). In this connection, the disk (37) simultaneously functions as a pressure lock. In an embodiment schematically represented in FIG. 4 as a top view, a disk (39) with several openings (41) that are filled with the reaction material (2) in tablet form functions as a pre-formulated storage vessel for the transport of the tablets over the heating unit (9). The storage vessel can be configured in the form of a customary blister packaging for tablets for example.
Plastic, for example PTFE or PFA, or glass is the preferred material for the disks (37) and (39).
In order to control automatic production of phosgene, an electronic regulation device can be used that displays and regulates the delivery of reaction material from the storage vessel, the temperature in the reaction chamber, the return flow device from the reflux condenser and heat exchanger and the gas exhaust over a valve.